- /metrics: atomic counters for requests, cache hits, MS fetches, neg
hits, stale serves, and cache sizes per endpoint. Auth via
METRICS_SECRET env var (query param or Authorization: Bearer).
- Cache eviction: cleanupCaches() goroutine runs every 30min, removes
expired SKU/neg entries and link entries past ExpiresAt + 4h stale
window. Prevents unbounded map growth under sustained traffic.
- rand: replace global rand with package-level rand.Rand to avoid
mutex contention at high QPS (jitter() uses local source).
- Singleflight keys: separate sfKey from negKey in both handlers for
clarity — "sku:<id>" and "link:<id>:<sku>" vs neg cache keys.
- Background refresh on stale: when /proxy serves a stale entry after
a failed refresh, fires a goroutine to retry and update the cache
so the next request gets fresh data.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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